Typical physical networks contain several physical routers to perform L3 forwarding (i.e., routing). When a first machine wants to send a packet to a second machine located on a different IP subnet, the packet is sent to a router that uses a destination IP address of the packet to determine through which of its physical interfaces the packet should be sent. Larger networks will contain multiple routers, such that if one of the routers fails, the packets can be routed along a different path between the first machine and the second machine. Both within a contained network and across network boundaries, routing protocols are used to advertise routes through the network. That is, a first router peers with a second router and sends messages to the second router indicating which addresses it can reach through its other interfaces and how far away those addresses are. The first router also receives corresponding information from the second router, and uses this information to determine how to route packets.
In logical networks implemented in a datacenter, user-defined data compute nodes (e.g., virtual machines) on different subnets may need to communicate with each other, as well as with machines external to the datacenter. In this case, tenants may define a network for virtualization that includes both logical switches and logical routers. Methods for implementing the logical routers to adequately serve such virtualized logical networks in datacenters are needed, including methods that allow for a similar route exchange with routers of the external physical network.